1. Field of the Invention
The present invention relates to an apparatus for optically reading a target including an information code.
2. Description of the Related Art
A conventional optical information reader aims at reading a target including information that is optically readable, such as a barcode, a two-dimensional code, or other similar codes.
FIG. 8 is a view schematically illustrating a basic configuration of a handheld two-dimensional code scanner as an example of a conventional optical information reader.
In the two-dimensional code scanner shown in FIG. 8, a user's handheld case 1 has at its one end portion with a rectangular reading window 1a. In the case 1, a lighting unit 2 composed of a plurality of light emitting devices (LED) is installed for emitting light through the reading window la toward an outside of the case 1. In the case 1, a reflector 3, an imaging lens 4, and a photodetector 5, such as a CCD area sensor, are installed, respectively. The two-dimensional code scanner is also provided with a processing circuit 6 in the case 1 for decoding a signal corresponding to a reflected light from the target and transmitted from the photodetector 5.
In order to optically read a two-dimensional code, such as a QR code written on a surface of a target 7, with the use of the scanner, a user brings the scanner toward the target 7 so that the scanner's reading window la is close to the target 7 in substantially parallel to the surface of the target 7. The user gives instructions for executing reading operations to the scanner so that light is emitted from the lighting unit 2 and the emitted light is irradiated through the reading window 1a to the target 7.
Light reflected from the target 7 is entered through the reading window 1a into the reflector 3 to be reflected toward the imaging lens 4. The reflected light enters into the imaging lens 4 and the entered light is focused on the active area (light sensitive pixel area) of the photodetector 5 by the imaging lens 4, so that an image corresponding to the target 7 is picked up by the photodetector 5. The image picked up by the photodetector 5 is sent to the processing circuit 6 so that the two-dimensional code of the label P is decoded by the processing circuit 6 based on the image. The conventional imaging lens 4 is composed of a minification optical system having a feature such that the more away an area that the lens 4 can view is from the lens 4, the wider an angle of view (a field of view) is.
Such a conventional optical information reader (referred to as information reader) is in common use for optically reading, as the target 7, a label, which is a piece of paper or another media, on which an information code is written. Recently, however, new usage that the conventional information reader optically reads an information code displayed on the screen of a liquid crystal display (LCD) in a computer terminal, such as a cellular phone or a PDA (Personal Digital Assistant), has been considered. In addition, users want to use such a conventional information reader for optically reading an information code directly printed on metal in direct marking.
When optically reading, as the target, the information code displayed on the screen of an LCD with the use of the conventional information reader, because the liquid crystal layer of an LCD has a very low transmittance to light, for example the order of a few percents, the emitted light is difficult to enter through the liquid crystal layer. The light reflected from the information code is also difficult to be transmitted through the liquid crystal layer. These reasons cause the amount of the reflected light that returns to the information code reader to decrease.
In particular, as shown in FIG. 8, if the optical axis A1 of the lighting unit 2 is inclined at an angle α with respect to a normal line A2 of the liquid crystal layer, it would be difficult to accurately read the information code displayed on the screen of an LCD.
Based on the background, a reading apparatus is designed so that the optical axis of a surface light source as a lighting unit is arranged to be orthogonal to the optical axis of a minification lens system and that of a photodetector. The reading apparatus is also designed to have a half mirror disposed between the lens system and the surface light source so that the optical axis of the half mirror is crossed at an angle of 45 degrees with respect to the optical axis of the lens system and that of the surface light source, respectively. The configuration of the reading apparatus is disclosed in Japanese Unexamined Patent Publication No. 2000-298698.
In the configuration of the disclosed reading apparatus, light emitted from the surface light source is reflected by the half mirror, and the reflected light is transmitted coaxially with the lens system toward a target. Light reflected from the target is transmitted through the half mirror to enter into the lens system, and the entered light is focused on the active area of the photodetector.
The configuration of the reading apparatus disclosed in the publication is provided with the minification lens system having a feature such that the more away an area that the lens system can view is from the lens system, the wider an angle of view is. The minification lens system of the reading apparatus disclosed in the publication can accurately read the information code displayed on the screen of an LCD if the screen-displayed information code is disposed close to the reading window of the reading apparatus.
If, however, the reading window of the reading apparatus is located away from the screen-displayed information code, the more away the screen-displayed information code is from the reading window, the smaller a magnification of the information code is. This may cause the number of pixels in the active area that are allocated to the image focused thereon to decrease. As a result, the disclosed reading apparatus may be difficult to read the screen-displayed information code if the reading window of the reading apparatus is located away from the screen-displayed information code.
In addition, in the configuration of the reading apparatus disclosed in the publication having the half mirror, the light emitted from the surface light source is diffused before the emitted light is irradiated to the target, causing light loss to increase. In particular, the quantity of light irradiated to the target may decrease to 30 percent of that of light emitted from the surface light source. This increase of the light loss may cause shortage of the quantity of light irradiated to the target.